This invention relates to a feed screw device of a ball screw, square thread, etc. Particularly, the invention relates to a feed screw device that can automatically supply a lubricant over a long term.
A conventional ball screw, a kind of feed screw device, is described in Japanese Utility Model Unexamined Publication Nos. Hei 7-4952 and 6-47762, etc., for example.
This kind of ball screw comprises a spiral thread groove 50a made in the outer peripheral surface of a screw shaft 50 threadably engaging a spiral thread groove 51a made in the inner peripheral surface of a nut member 51 via a plurality of balls 52, as shown in FIG. 1, for converting relative rotation of the screw shaft 50 to the nut member 51 into relative displacement in an axial direction of the nut member 51 via the balls 52.
An annular recess 53 is formed in both end parts of the inner diameter face of the nut member 51 (in FIG. 1, only the right end is shown), and a sealing member 54 is mounted on the recess 53.
The sealing member 54 is made from plastic containing a lubricant into a ring shape, and a projection 54a that can be fitted into the thread groove 50a of the screw shaft 50 projects from the inner peripheral surface of the sealing member.
A ring-like garter spring 55 is inserted in the circumferential direction between the outer peripheral surface of the sealing member 54 and the recess 53 of the nut member 51. The garter spring 55 clamps down all the outer peripheral surface of the sealing member 54 toward the outer peripheral surface of the screw shaft 50, that is, the sealing member 54 is pressed in the inner diameter direction.
Further, a tapped hole 56 radially penetrating the circumferential wall of the nut member 51 is made in the position of the recess 53 in the nut member 51 and a set screw 57 is fitted into the tapped hole 56, thereby fixing the sealing member 54 to the nut member 51.
The garter spring 55 sets the gap between the inner peripheral surface of the sealing member 54 and the outer peripheral surface of the screw shaft 50 to zero or less for preventing the lubricant filled in the ball screw from leaking to the outside and a foreign material from entering the ball screw from the outside.
Further, the lubricant exuding from the sealing member 54 decreases frictional resistance of the slide portion between the inner peripheral surface of the sealing member 54 and the outer peripheral surface of the screw shaft 50, namely, slide torque and at the same time, is supplied to the thread groove 50a of the screw shaft 50, the balls 52, and the thread groove 51a of the nut member 51.
However, for the conventional feed screw device of the structure as described above, the garter spring 55 needs to be inserted between the outer periphery of the sealing member 54 and the recess 53 of the screw shaft 50 so that a dimension error of the sealing member 54 is absorbed and that the inner peripheral surface of the sealing member 54 comes in sliding contact with the outer peripheral surface of the screw shaft 50; it is feared that the outer diameter of the feed screw device may grow as large as the garter spring.
In the conventional structure, the sealing member 54 is pressed diametrically, so that it is abutted against the outer peripheral surface of the screw shaft 50, thus the spring needs to be disposed on all the outer periphery of the sealing member 54 in the circumferential direction.
The inner diameter portion on the opposite side (portion not pressed by the garter sprint) to the side pressed by the garter spring 55 of the sealing member 54 in the axial direction apts to float-up with respect to the outer peripheral surface of the screw shaft and it is feared that a sufficient lubricant will not be supplied from the portion.
Some conventional feed screw devices comprise a spring inserted between the tip of the set screw 57 and the sealing member 54 in place of the garter spring 55. However, the lubricant supply section is also pressed only diametrically, thus it is feared that the inner peripheral surface of the sealing member 54 may be placed out of contact with the outer peripheral surface of the screw shaft 50 or that the portion pressed by the set screw in sliding contact with the screw shaft may be worn on one side in portions shifted 90 degrees from the placement position of the set screw 57, etc., for example.
Even if hardware for suppressing jump out of the sealing member 54 from the recess 53 is attached to the nut member 51, the tapped hole 56 needs to be made in the nut member 51 and fixed by the set screw 57 to prevent accompanying rotation of the screw shaft 50 of the sealing member 54 in addition to the hardware; workability is poor.
Further, when the sealing member 54 has a part cut (for example, the sealing member 54 is made into c-shaped) because of attachment thereof, the inner peripheral surface of the sealing member 54 does not come in sufficient contact with the screw shaft 50 and it is feared that a sufficient lubricant will not be supplied from the sealing member 54 depending on the operating condition.
Known as another ball screw, a kind of conventional feed screw device, is a device wherein a space 204 between a screw shaft 201 and a ball screw nut 202 threadably engaged into the screw shaft 201 via a large number of balls 203 is filled with grease or lubricant, as shown in FIG. 2. The ball screw is of seal type wherein an annular recess 205 is made in an end of the ball screw nut 202 and a sealing member 206 is fitted into the annular recess 205 to prevent powder dust, etc., from entering into the ball screw nut 202.
By the way, to lubricate such a conventional ball screw device, normally grease or lubricant is supplied from a grease nipple 207 attached to the ball screw nut 202 to a ball circulation passage for lubricating a rolling element.
However, particularly when the ball screw device adopting such a lubrication system directly using lubricant or grease is used in a high-temperature environment or a environment wherein wood chips, etc., easily absorbing lubricants are existed, the filled lubricant or grease flows out to the outside, is exhausted fast, and must be again supplied repeatedly for a short term. Japanese Utility Model Unexamined Publication No. Hei 7-4952 is known as application relating to an oil-containing polymer lubrication ball screw to improve this point.
For the oil-containing polymer lubrication ball screw disclosed here, a lubricant supply member mounted on a ball screw nut is formed of lubricant-containing rubber or synthetic resin and the lubricant continuously exuding from the lubricant supply member is automatically supplied to a rolling element lubrication passage of the ball screw nut.
However, with the ball screw, as the lubricant supply member containing the lubricant runs with the ball screw nut while coming in contact with the screw shaft, the lubricant exudes from the contact part for lubrication; lubricant supplied to a guide rail is easily absorbed particularly in an environment wherein foreign materials such as wood chips easily absorbing lubricant are existed, and the lubricant can also be absorbed from the lubricant-containing polymer member; resultantly, it is feared that a lubrication failure may be invited.